Design Patterns – Iterators, Observers, and Chains, Oh My

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We’re back to the gang of four, continuing with another segment of design patterns. This time we’re talking about some of our favorite Behavioral Design Patterns: Observer, Chain of Responsibilities, Iterator patterns. Also, why the visitor pattern is weird and what it’s like to be raked over hot coals.

News

• Atlanta Code Camp was amazing – $10 for all of this!
• AngularJS is HUGE, despite the recent controversy
• Ionic – Cross platform mobile development
• Bang Bang Javascript – Reddit will beat you up
• Codecademy – very cool way of learning new programming skills!
• The visitor pattern is wha,cray,wei@$@af–,what?

The Observer Pattern

• Abstracts message senders and receivers from each other
• Watch out for memory leaks!
• Be careful with abstraction layer hopping and event overloading!
• Examples in .NET
  • Events
  • IObserver and IObservable

Chain of Responsibilities

• Examples in .NET
  • Exceptions – Sorta
  • HTTP Handlers – Sorta
  • MVC Routing – Sorta
• Decouples the sender from the receiver – allows more than one object a chance to handle it
• Differences between Chain of Responsibilities and Intercepting Filters Patterns
• Differences between Chain of Responsibilities and Observer Patterns

Iterator Pattern

• Decouples algorithms from containers – good for data structures…but especially for sequences, algorithms
• When to use it:
  • Sequentially traverse a collection
  • Decouple the traversal logic/algorithm from the container
  • Dealing w/ sequences – like weird enumerations! Fibonacci sequence
• Examples in .NET
  • foreach (operates on IEnumerable types)
  • LINQ
  • yield keyword
  • IEnumerable – aggregate/collection/container interface
  • IEnumerator – iterator interface returned from IEnumerable
  • Iterators tracks it’s state internally, so beware of concurrency / parrallelization issues

Resources we like

• Codecademy
• Gang of Four Book
• Design Patterns on Pluralsight

Tips / Tricks

• Rextester Share code Snippets in the browser
• The Postman – Test REST in Chrome
• jsPerf – Performance testing for Javascript

Thanks for listening!